1. Field of the Invention
The present invention relates in general to an image sensor, a reading device and a method for setting a resolution. In particular, the present invention relates to an image sensor, a reading device with the image sensor and a method for setting a resolution of the image sensor, wherein the image sensor comprises a plurality of photoelectric converting elements for converting light signals into electric signals, and a group of channel select switches capable of being sequentially turned on and off to control a connection between a charge output unit of the photoelectric converting element and a common signal line.
2. Description of Related Art
Image sensors are used in a reading device, such as a facsimile, a copy machine and a hand scanner, etc. FIG. 14 shows a structure of a conventional image sensor. Referring to FIG. 14, photoelectric converting elements P1a˜P1e are phototransistors, etc. for detecting light to output electric currents (image signals). P2 is a power input terminal for inputting a power voltage VDD. P3a˜P3e are channel select switches respectively connected to charge output units of the photoelectric converting elements P1a˜P1e. P4 is a shift register group, wherein the shift register group P4 is activated by a start signal, image signals output from the photoelectric converting elements P1a˜P1e are sequentially output from an image signal output terminal P11 through a common signal line P7, and each channel select switch P3a˜P3e can be sequentially turned on and off according to the period of a clock pulse signal. P4a˜P4f are shift registers. P5 is a start signal input terminal for inputting the start signal (SI). P6 is a clock pulse signal input terminal for inputting the clock pulse signal (CLK).
After activated by the start signal SI, a flip-flop P8 continuously outputs an “ON” signal during an operation period of the shift register group P4, a period starting from inputting a start signal SI to the shift register P4 a until the start signal SI is output from the shift register P4f. A chip select switch P9 is inserted onto a signal line P7, so that the chip select switch P9 is turned off upon receiving the “ON” signal from the flip-flop P8. A switch P10 is connected between the signal line P7 and a ground terminal P12 to repeatedly open and close according to a level change of the clock pulse signal CLK.
Next, the operation of the conventional image sensor is described. The start signal SI and the clock pulse signal are externally provided to the shift register group P4 through the start signal terminal P5 and the clock pulse signal input terminal P6, respectively. The period of start signal SI is twice of the period of the clock pulse signal CLK, and is read by the shift register P4a of the shift register group P4 at the falling edge of the clock pulse signal CLK.
The shift register P4a is activated by reading the start signal SI, and then the shift register P4a closes the channel select switch P3a for a time interval of one period of the clock pulse signal CLK. In this way, the image signal output from the photoelectric converting element P1a is output from the image signal output terminal P11 through the signal line P7. Then, the channel select switch P3a returns to its open status and the read start signal SI is transmitted to the shift register P4b. 
Therefore, since the start signal SI is read by a shift register sequence of P4b→P4c→P4d→P4e, the image signals of the photoelectric converting elements P1b to P1e are sequentially output from the image signal output terminal P11. In addition, the output of the shift register P4f is transmitted through the terminal P13 to serve as a start signal for the next-stage sensor IC.
In the aforementioned image sensor, the control signal externally input is used to selectively switch and output the current, which is output from the photoelectric converting element to the image signal output terminal, by determining whether the control signal is a high level (H) or a low level (L), and the resolution for reading is set to two levels. Refer to Japanese Laid Open Publication H05-227362.
However, in the above method for setting the resolution, the resolution can be only set to two levels, a resolution corresponding to when the control signal is “H” and a resolution corresponding to when the control signal is “L”. In contrast, most of general reading devices have to set the resolution to multi-levels, exceeding two levels. If control signals with plural types, such as two types, three types, four types, . . . etc., can be input, the resolution can be set to multi-levels, such as four levels, eight levels, sixteen levels, . . . etc. However, in this case, it needs a lot of additional signal lines, and thereby substantially increasing the cost.